Writing a plugin

Here’s another guy trying to learn how to write a plugin. I am VERY new to writing a plugins and this is the only method in which I can get my function to work.

In the world of plugins, is it ok to write a function like this.

function my_function()
{
   ?>  

<div id="box">

<div id="myLoop">
<?php query_posts('showposts=5');?>
          <?php while (have_posts()) : the_post(); ?>
          <div>
                    <div class="title"><a href="<?php the_permalink() ?>">

                              </a></div>
                    <a href="<?php the_permalink() ?>" rel="bookmark" title="Link to <?php the_title(); ?>">
                    <h3><strong><?php the_title(); ?></strong></h3>
                    </a>
                  <?php the_excerpt(); ?>
          </div>
          <?php endwhile; ?>
          //Reset Query
<?php wp_reset_query(); ?>

</div>
</div>
<?php
 }
 ?>

And then include the function on a page with:

<?php
if (function_exists("my_function")) {
  my_function();
}
?>

Should I code this differently when working with pluin API?

Thanks for any help.